Strong eye contact is essential for effective communication in headshots and to get the attention of the casting director. Looking straight down the barrel of the lens or just above it is the best strategy. Most people also have one eye that is slightly smaller or a different shape than the other and unsymmetrical facial features. This is why most of us have a 'good' side and a 'bad' side. Being photographed in 3/4 profile overcomes this problem.
In an effort to look open and interesting, some clients try opening their eyes as wide as they can. This gives a look which at best is unnatural and at worst is just plain creepy.
We suggest you try and laugh with your eyes. A good way to achieve this effect is to squint ever so slightly with your lower eye lids only. Don't over do the squinting - otherwise you will look like Clint Eastwood ready for a shoot out.